Catheters which include a variety of electrode means for the ablation of biological tissue are known. For example, most of the catheters used for the ablation of cardiac tissue employ electrodes which are energized by a radio-frequency ("RF") electrical current. That is, the cardiac tissue is heated by the RF-energized electrodes until a lesion is formed in the desired location. Generally, the RF-electrode catheters are also used to map, or to record, the electrical conduction pathways of the cardiac tissue.
Examples of such RF-energized electrode catheters for use in the treatment of cardiac arrhythmia appear in U.S. Pat. No. 5,720,775 to Larnard; U.S. 5,676,693 to LaFontaine; U.S. Pat. No. 5,555,883 to Avitall; U.S. Pat. No. 5,545,193 to Fleischman et al.; U.S. Pat, No. 5,281,213 to Milder et al.; and U.S. Pat. No. 5,281,215 to Milder.
Generally, in the treatment of tachycardia, electrode mapping is used to locate the originating site of the disturbance in the electrical activity or rhythm of the heart. A focal lesion is formed at the originating site of the arrhythmia to interrupt the abnormal electrical activity. Generally, in the treatment of atrial fibrillation, electrode mapping is used to record the electrical conduction system of the atria in order to ensure that the ablation electrode does not reach the normal conduction system. In such treatment, elongated, continuous lesions are formed in the atrial tissue to block reentry circuits which disrupt the normal electrical activity of the atria.
Use of RF-energized electrode catheters to form such cardiac lesions has significant disadvantages. For example, often when using such a catheter to form a cardiac lesion, the cardiac tissue becomes charred from the RF-energized heating of the tissue, blood in the vicinity of the cardiac tissue undergoing treatment becomes coagulated, and the cardiac tissue undergoes separation and/or popping. Further, it is often difficult to form an effective lesion having sufficient length, continuity and/or depth to interrupt or to block electrical conduction across the lesion, when using an RF-energized electrode catheter.
Theoretical consideration and experimental testing have indicated that the formation of cardiac lesions by cryogenic means overcomes many of the disadvantages associated with the use of RF-energized electrode catheters. Yet, cryogenic means have not become the means of choice, for lack of development of a desirable and effective cryogenic system and technique for the treatment of cardiac conditions.
By way of example, Milder (above) discloses a cryogenic catheter for performing "ice-mapping" and cryogenic ablation of cardiac tissue. The catheter is designed for cryoablation at its tip, such that focal lesions are formed. The catheter of Milder is not suitable for the formation of the elongated, continuous, and/or deep lesions that are desirable in the treatment of various cardiac conditions, such as atrial fibrillation.
There is a need for a cryogenic system, and a method of using same, that provides for efficacious treatment of biological tissue by forming an efficacious lesion in the tissue. There is a particular need for same for the treatment of abnormal conditions in cardiopulmonary tissue.